Backpack



FIG. 1 is a front elevational view of a backpack, showing my new design;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a left side elevational view thereof;

FIG. 4 is a right side elevational view thereof;

FIG. 5 is a bottom plan view thereof; and,

FIG. 6 is a front perspective view thereof.

The zipper and the zipper pull, which are shown in phantom, and the surface texture of the backpack, are not part of my claimed design. 

The ornamental design for a backpack, as shown and described. 